Hot fluid injection into hydrocarbon reservoirs

ABSTRACT

Hot hydrocarbon solvent may be injected into a subterranean hydrocarbon reservoir with little heat loss through the casing walls on the way down to the injection zone by placing tubing inside the casing of an injection well and injecting a hot fluid or vapor into the tubing and a solvent fluid into the annulus between the tubing and casing.

HOT FLU/0* iJmted States Patent 1191 1111 3,837,401 Allen et al. 1 1 Sept. 24, 1974 HOT FLUID INJECTION INTO 3,186,484 6/1965 Waterman 166 272 x HYDROCARBON RESERVOIRS 3,221,813 12/1965 Closmann et 166/272 X 3,380,530 4/1968 McConnell et al. 166/303 Inventors: J p Allen; Ylck-Mow Shum, 3,386,512 6/1968 13166111 166/303 both c/o Texaco, Inc., PO. Box 3,456,730 7/1969 Lange 166/272 X 425, Houston, Tex. 77401 3,498,381 3/1970 Earlougher, Jr. 166/303 3,559,738 2 1971 S '11 t 166 303 22 Filed: July 12, 1973 e [21] Appl. No.: 378,474 Primary Examiner-Stephen J. Novosad Related Us. Application Data Attorney, Agent, or Fzrm-T. H, Whaley; C. G. Rles [63] Continuation-impart of Ser. No. 229,927, Feb. 28,

1972, Pat. No. 3,774,684. [57] ABSTRACT Hot hydrocarbon solvent may be injected into a sub [52] US. Cl. 166/303 terranean ydr s r oir w th littl heat loss [51] Int. Cl E21b 43/24 hrough the casing walls on the way down to the injec- [58] Field of Search 166/303, 272, 302, 305 R tion zone by placing tubing inside the casing of an injection well and injecting a hot fluid or vapor into the [56] References Cit d tubing and a solvent fluid into the annulus between UNITED STATES PATENTS the tubmg and 998mg- 1,237,139 8/1917 'Yeomans 166/272 X 7 Claims, 3 Drawing Figures PAIENTEBSEFZMQH HOT FLU/Dfi Hor FLU/D EXTRA/V5005 FORMA r/o/vs H Ybko CARBON v cow FLU/D FCOL FLU/Dy I I; I

BEARING =71 RESERVOIR 'EXTRANEOUS FORMA T/ONS BEARING RESERVOIR COMPARA T/ VE HEAT LOSSES I TIME (h) HOT FLUID INJECTION INTO HYDROCARBON RESERVOIRS This is a continuation-in-part of Application, Ser. No. 229,927, filed Feb. 28, 1972 and now US. Pat. No. 3,774,684.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is concerned with the field of secondary recovery of hydrocarbons from subterranean reservoirs by the injection of a hot solvent for hydrocarbons into the reservoir.

2. Discussion of the Prior Art In many hydrocarbon producing areas there are reservoirs where production is no longer commercially feasible due to the fact that the original pressure in the hydrocarbon stratum has been exhausted to the extent that hydrocarbons will no longer move through the formation into production wells in sufficient quantities to permit profitable operation. Usually, however, these reservoirs in fact have more oil remaining in them than has been produced.

Also, many reservoirs contain very viscous hydrocarbons. This high viscosity impedes flow and much of these viscous hydrocarbons are trapped in the reservoirs even while relatively high reservoir pressures remain. Attempts to recover these hydrocarbons have included the injection of a fluid into the reservoir to increase the pressure in the reservoir and displace the hydrocarbons to production wells where they are produced. These fluids are typically aqueous, gaseous, some hydrocarbon or a mixture of materials. It has been recognized that the temperature of the injected fluids have a great influence on the efficiency of the recovery process. The hydrocarbons in the reservoir will flow more readily if they are heated by the injected fluid with a consequent reduction in their viscosity. Therefore, it is often necessary and more efficient to inject a hot fluid into the reservoir.

However, in the past, cost of heating the great volumes of injection fluids has been inflated because of the great losses of heat from the hot injection fluid through the sides of the well into extraneous formations on the way down the injection well bore to the hydrocarbon formation of interest. Thus, it has previously been necessary to heat the injection fluids at the surface to much higher temperatures than is desired at the injection point. Attempts have been made to decrease the amount of injection fluid heat loss as it travels down the injection well to the point of injection. For instance, tubing may be installed in the injection well with devices to centralize it in the well bore and prevent its contact with the casing where rapid heat conduction would increase the flow of heat from the injection fluid. However, much heat is still lost through the tubing walls into the annular spaces between the tubing and casing and then to extraneous formations in the earth. Other methods such as insulation have also been attempted but leave much to be desired because of the expense involved.

Also, due to depths of some hydrocarbon formations pressure limitations are placed on surface injection facilities which prevent hot enough fluidfrom being injected at the surface. The term hydrocarbon solvent is used interchangeably hereinafter to denote a solvent for hydrocarbons whether or not the solvent is itself a hydrocarbon.

SUMMARY OF THE INVENTION Our invention is a method of injecting a hot hydrocarbon solvent into a subterranean hydrocarbon reservoir with a minimum of heat loss from the injected fluid to the extraneous formations which comprises providing tubular means inside of well casing and injecting BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts an arrangement whereby an open ended string of tubing penetrates a well wherein hot fluid may be injected into the tubing and cold hydrocarbon solvent into the annular space between the tubing and the casing.

FIG. 2 compares the heat lost to the formations above the hydrocarbon zone of interest using prior art hot solvent injection techniques and the method of our invention as illustrated by FIG. 1.

FIG. 3 depicts an arrangement wherein a closed tubing arrangement is provided in the well so that a hot fluid may be circulated into the well and out without contacting the cold hydrocarbon solvent injected into the annular space between the tubing and the casing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Our invention may be more clearly understood by referring to the attached figures which illustrate typical embodiments-of our invention. FIG. 1 shows an injection well penetrating the earth with perforations in communication with a hydrocarbon bearing reservoir. A single string of tubing 10 penetrates the well to a depth above the perforations 11. The tubing is open at the bottom and is therefore in communication with the other materials in the well. Toluene vapor or steam, for example, is injected into the tubing and cold toluene, for example, is injected into the annular space between the tubing and casing. The rates of injection are adjusted so that liquid toluene in the annular space heats up slowly and reaches the desired temperature for injection just as the toluene reaches the depth of the perforations. In this manner a minimum of heat will belost to extraneous formations above the hydrocarbon bearing reservoir of interest. The heat of the toluene vapor and steam is used to heat the liquid toluene in the annulus, and so longas the toluene in the annulus remains cooler than the subterranean ambient temperatures no heat will be lost. In fact, heat will be added to the toluene in the annulus from the surrounding formations in the earth. Only when the temperature of the toluene in the annulus exceeds the temperature of the surrounding formations will any heat be wasted. By heat conduction calculations known to those skilled in the art it will be possible to calculate the rate of toluene vapor or steam and liquid toluene injection to allow the toluene in the annulus to travel the maximum distance into the earth before its temperature exceeds the temperature of the surrounding formations in the earth. In this way a minimum amount of heat will be lost to extraneous earth formations. It is clear that this method provides a great savings in heat energy and thus a corresponding economic advantage as shown by the following illustration.

The total heat delivered to the hydrocarbon formation by liquid toluene-steam injection was calculated for the theoretical situations as noted below.

Case I (prior art) Saturated steam, at 1,000 psia, 545F., 80 percent quality and 270 barrels per day (BPD), was mixed with water, at 1,000 psia, 70F., and 260 BPD, adiabatically. The resulting mixture, approximately 3 percent quality steam at 545F., 1,000 psia, was injected into 2 /8 in. tubing suspended at 4,000 in a well with 7 inch casing.

Case II (water in annulus-steam in tubing) Using the same tubing and casing diameters, the heat delivered to the formation was calculated using the same steam and water properties given in Case I. However, in this case only steam was injected into the tubing and the water was injected into the tubing-casing annulus.

Case 111 (toluene in annulus-steam in tubing Again using the same tubing and casing diameters, the heat delivered to the formation was calculated using the same steam injected into the tubing but with toluene injected down the annulus. The toluene is injected at the rate of 260 BPD at 70F. initial temperature.

Results for each case are represented in FIG. 2 by plotting the heat lost to the overburden formation as a function of time. The heat loss from the well bore to the overburden formation was determined by standard mathematical techniques using a computer program. Theory and development of the calculation procedure is based on a quasi-steady-state method. Case II and Case Ill results were developed by standard mathematical techniques using a computer program. The energy is transferred from the hot fluid to the cold fluid in the annulus, and, subsequently to the overburden formations above the formation which accepts the injected fluid. FIG. 2 shows that the heat loss to the extraneous formations would be cut approximately in half using the method of our invention. Due to the transient nature of the problem, the principle of superposition was used to determine the thermal energy transferred from the fluid in the annulus to the overburden formations. Solutions were obtained using established energy balance equations with numerical iteration. Briefly the solution procedure was as follows:

1. Assume a T,,,,, the outlet temperature of the injected steam at a rate of S (lb/hr).

Due to the large values of the convective heat transfer coefficient and well depth, H (ft), the outlet temperature of the injected fluid, T would be equal to T 2. Thermal energy transfer calculation:

Thermal energy given up by the steam is Q8 T S SH X (BTU/hr) where H is the latent heat of the injected steam, (BTU/lb), and

X is the inlet steam quality.

Thermal energy gained by the injected fluid in the annulus is QW wo wi) W where W is the fluid injection rate (lb/hr).

3. Thermal energy transfer from the fluid in annulus to the surrounding formation by'conduction, Q,, is calculated as follows:

log A a a 1og B a log B a log B a.,.

where A R F/KT, dimensionless heat flux. B at/R 2, dimensionless time, a a a a and a are constants, R is the casing radius, ft. K and a are the thermal conductivity (BTU/hr-ftF.) and diffusivity (ft /hr), respectively. t is time, hr. T is the temperature difference between the casing surface and the formation, F. F is the heat flux, BTU/hr-ft Then Q,= FA (BTU/hr) 4 where A is the casing surface area, ft Due to the transient nature of this problem, the method of superposition must be employed as illustrated as follows:

a. At the end of the 1st time step, say t wo wo! 1, ami 1 wi wo. 1) T is the average fluid temperature. T]: 0.5 (Tn Tm) where T is the ground surface temperature and T is the sand face formation temperature, F. F is calculated from Equation 3 with T= T 1 7}, and

t=t b. At the end of the 2nd time step, i.e.,

If equation (5) cannot be satisfied within a preset tolerance, a new value of T is selected and calculations from steps 1 to 4 are repeated. Otherwise, solution is advanced one time step by starting the calculation from step 1.

Some conditions may exist, however, which would prevent the convenient operation of the procedure illustrated by FIG. 1 above. FIG. 3 illustrates a procedure which is another typical embodiment of our invention. FIG. 3 is a closed tubing arrangement wherein all the benefits embodied in the open tubing arrangement of FIG. 1 are provided with the added feature of segregation between the hot fluid in the tubing and the cold hydrocarbon solvent to be heated in the annular space between the tubing and casing. A large string of closed end tubing 10 penetrates a well 11 and a smaller string of open end tubing 12 penetrates the closed end string of tubing. A hot fluid, steam, for instance, is circulated down through the annular space 13 between the tubing strings and up through the small inside tubing string. A cold hydrocarbon solvent is injected down the annular space 14 between the large tubing strings and the well casing. This solvent is heated on the way down the well bore in the manner described in FIG. 1 by contacting the heated tubing string 10 and is injected into a hydrocarbon reservoir. Once again injection rates may be adjusted so that the solvent heats as slowly as possible and attains its desired temperature at the depth of the hy-' drocarbon formation in which it is to be injected. The method of FIG. 3 may be used in shallow formations where the temperature of an injected vaporized solvent is limited to the injection pressure which is in turn limited by the shallow depth. Since injecting solvent vapor at an excess pressure to provide high temperature might lift the overburden thus rupturing the earth formations with harmful consequences known to those in the art of oil production, the open tubing arrangement of FIG. 1 cannot be desirable since the pressure of the solvent vapor is communicated to the earth formations. Also, where it is desired to inject superheated hydrocarbon solvent vapor into a shallow formation the method of FIG. 3 would provide superheated solvent vapor at the hydrocarbon reservoir depth while injecting only saturated steam or solvent vapor at the surface. Thus, in shallow formations high pressure which may rupture the formations will not have to be used in order to attain high fluid injection temperatures since the high pressure fluid in the closed circulating tubing arrangement is isolated from the formation. Another example with the open tubing embodiment of FIG. 1 would be inapplicable is where the hydrocarbon reservoir is very deep. Here the solvent vapor would be condensed to liquid for a considerable depth in the well. Thus, very high pressure would be necessary in order to maintain the desired rate of hydrocarbon solvent vapor injection.

Also, the method of our invention allows a hotter fluid to enter the shallow formation than would be possible by injecting hot fluid from the surface. For example, consider a reservoir 600 feet below the surface. By a typical standard practice the overburden at 600 feet is calculated to be 600 pounds per square inch or 1 pound per square inch per foot of depth. Above this pressure the formation is likely to fracture or rupture. Thus, the maximum surface injection pressure of the solvent in the annulus is limited and, so therefore, is the bottom hole pressure and temperature. By using the technique depicted in FIG. 3 a fluid could be injected into the tubing strings and out at a temperature and pressure not limited by the overburden pressure; since the pressure in the tubing strings is not communicated to the formation. Thus, the fluid in the annulus could be heated to a much higher temperature than would otherwise be possible.

Also, the basic advantage of our invention, the conservation of heat energy is also gained. The above example assumes for illustration purposes no heat loss of the solvent in the annulus injected at the surface when in fact there would be heat loss on the way to the formation. When this is taken into account an even greater advantage would be realized by using the method of our invention.

The hydrocarbon solvents useful in the method of our invention encompass the whole range of solvents which have miscibility with reservoir hydrocarbons. Lease crude may be used as well as pure aromatic and- /or aliphatic materials. Nonhydrocarbons are also use ful. Examples of suitable solvents include LPG, butane, propane, pentane, hexane and'their homologs. Also benzene, xylene, toluene, naphthas and their derivatives are also useful. Solvents such as carbon disulfide and chlorinated hydrocarbons are also suitable for the process of our invention. Other mechanical arrangements are those shown in FIGS. 1 and 3 may be envisioned and still be within the scope of our invention. The types of reservoirs and hydrocarbons to which our invention is best suited are those in which hot fluids are more efficient than cold fluids for the recovery of hydrocarbons. Prior art is replete with information concerning hot fluid injection and its advantages.

We claim:

1. A method for injecting a first fluid into a subterranean hydrocarbon reservoir via a cased well penetrating and in fluid communication with the reservoir wherein there exists tubular means inside and open at some point to the annular space between the tubular means and the casing wall and said first fluid is a mixture of a second fluid injected down the tubular means and a hydrocarbon solvent injected down the annulus comprising 1 injecting said hydrocarbon solvent into the annulus which solvent is initially at a temperature below the reservoir temperature, injecting said second fluicl'into the tubular means at a temperature above the temperature of the reservoir before injection began in such a way that the cooler solvent in the annulus is heated by the fluid in the tubular means to a temperature in excess of the reservoir temperature at about the same time said solvent in the annulus reaches the depth of the hydrocarbon reservoir and injecting said first fluid into the hydrocarbon reser- VOII'.

2. The method of claim 1 wherein said first fluid entersthe reservoir as a liquid.

3. The method of claim 1 wherein said first fluid enters the reservoir partially or completely vaporized.

4. A method for injecting a hydrocarbon solvent into a subterranean hydrocarbon reservoir via a cased well penetrating the reservoir wherein there exists tubular means inside the well arranged to prevent fluid communication between the inside of the tubular means and the annular space between the tubular means and the casing wall and wherein said annular space is in fluid communication with the reservoir comprising injecting said hydrocarbon solvent into the annular space between the tubular means and the casing wall which liquid is initially below reservoir temperature,

injecting a second fluid into the tubular means at a temperature above the reservoir temperature before injection began in such a way that said cooler hydrocarbon solvent in the annulus is heated by said second fluid in the tubular means to a temperature in excess of the reservoir temperature at about the same time said hydrocarbon solvent in the annulus reaches the depth of the hydrocarbon reservoir and injecting said first fluid into the hydrocarbon reservoir.

5. The method of claim 4 wherein said hydrocarbon solvent enters the reservoir as a liquid.

6. The method of claim 4 wherein said hydrocarbon solvent enters the reservoir partially or completely vaporized.

7. A method for heating a hydrocarbon solvent inhydrocarbon bearing reservoir which comprises contacting said hydrocarbon solvent in the well bore with a fluid encased in said tubular means wherein said fluid is at a temperature greater than the temperature of the hydrocarbon reservoir and said hydrocarbon solvent is initially at a temperature less than the temperature of the hydrocarbon reservoir and wherein the hydrocarbon solvent exceeds the reservoir temperature at about the time the hydrocarbon solvent reaches the depth of the hydrocarbon reservoir.

37 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No'. 3,837,401 Dated September 24, 1974 Inventor) Joseph C. Allen and Yick-Mow Shum It is certified that error appears in the above-identified pat ent and that said Letters Patent are hereby corrected as shown below:

Title Page, after list of inventors, item [76] insert :A ssignee: Texaco Inc., New York, N. Y.

i Column 4, line 36, "(T T j" should read (Tgk Tw Signed and sealed this 14th day of January 1975.

(SEAL) Attest:

McCOY M. GIBSON JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents 

2. The method of claim 1 wherein said first fluid enters the reservoir as a liquid.
 3. The method of claim 1 wherein said first fluid enters the reservoir partially or completely vaporized.
 4. A method for injecting a hydrocarbon solvent into a subterranean hydrocarbon reservoir via a cased well penetrating the reservoir wherein there exists tubular means inside the well arranged to prevent fluid communication between the inside of the tubular means and the annular space between the tubular means and the casing wall and wherein said annular space is in fluid communication with the reservoir comprising injecting said hydrocarbon solvent into the annular space between the tubular means and the casing wall which liquid is initially below reservoir temperature, injecting a second fluid into the tubular means at a temperature above the reservoir temperature before injection began in such a way that said cooler hydrocarbon solvent in the annulus is heated by said second fluid in the tubular means to a temperature in excess of the reservoir temperature at about the same time said hydrocarbon solvent in the annulus reaches the depth of the hydrocarbon reservoir and injecting said first fluid into the hydrocarbon reservoir.
 5. The method of claim 4 wherein said hydrocarbon solvent enters the reservoir as a liquid.
 6. The method of claim 4 wherein said hydrocarbon solvent enters the reservoir partially or completely vaporized.
 7. A method for heating a hydrocarbon solvent injected into the most external annular space in an injection well bore containing tubular means wherein said annular space is in communication with a subterranean hydrocarbon bearing reservoir which comprises contacting said hydrocarbon solvent in the well bore with a fluid encased in said tubular means wherein said fluid is at a temperature greater than the temperature of the hydrocarbon reservoir and said hydrocarbon solvent is initially at a temperature less than the temperature of the hydrocarbon reservoir and wherein the hydrocarbon solvent exceeds the reservoir temperature at about the time the hydrocarbon solvent reaches the depth of the hydrocarbon reservoir. 